We report the femtosecond time-resolved dynamics of relativistic electron pulses in ultraintense laser-foil interactions, by characterizing the terahertz self-radiation with single-shot ultrabroadband interferometry. Experimental measurements together with theoretical modeling reveal that the electron pulses inherit the duration of the driving laser pulse. We also visualize the electron recirculation dynamics, where electrons remain trapped inside the self-generated electrostatic potential well and rebound back and forth around the thin foil for hundreds of femtoseconds. Our results not only demonstrate an in situ, real-time metrology scheme for electron bursts, but also have important implications for understanding and manipulating the time-domain properties of laser-driven particle and radiation sources.